The present application relates to a resin composition including a polyester capable of forming a crystal structure and a resin molded product obtained by molding the resin composition, and more particularly relates to a resin composition which has durability by promoting crystallization of a biodegradable resin.
In recent years, concomitant with an increase in environmental consciousness, the use of a resin material degradable under natural environmental conditions, that is, the use of a resin material having so-called biodegradable properties, has drawn attention.
Unlike publicly disclosed general-purpose resins, since a resin having biodegradable properties is manufactured by using, for example, a non-fossil fuel as a primary raw material, an adverse influence of shortage of raw materials caused by the depletion of resources is advantageously small. In addition, since being degraded in the natural environment, a resin having biodegradable properties advantageously serves to solve problems relating to waste treatment. Furthermore, a resin having biodegradable properties can be advantageously manufactured from natural resources such as corns. In addition, a resin having biodegradable properties can advantageously reduce the amount of CO2 gas which is one of causes of the global warming; hence, hereinafter, the material described above is expected to draw more attention.
Among biodegradable resins, for example, an aliphatic polyester, in particular, a poly(lactic acid), has a high melting point (170 to 180° C.) and also has superior material properties capable of forming a transparent molded product, and hence it is expected that the above aliphatic polyester will have wide practical utility.
The biodegradable resin described above has been primarily used, for example, for materials for agriculture, forestry, and fisheries (films, planting pots, fishing lines, fishnets, and the like); civil engineering work materials (water retention sheets, nets for plants, and the like); package and container materials (hard to be recycled due to adhesion of soil, food, and the like), and disposal goods, such as convenience goods, sanitary goods, play goods. However, in view of environmental conservation, a further increase in use of the biodegradable resin has been studied.
A biodegradable resin has been studied to be used for electrical and electronic products, such as chassis of televisions and housings of personal computers, and in consideration of the application to chassis and structural materials of electrical products as described above, in general, it is believed that a biodegradable resin be requested to have heat resistance at approximately 80° C.
However, since a biodegradable polyester has inferior heat resistance, and for example, since a poly(lactic acid), which is one representative example thereof, has a glass transition temperature (Tg) of approximately 60° C., a molded product obtained therefrom is softened and deformed at a temperature more than the glass transition temperature; hence, there has been a problem of heat resistance in practical use.
In addition, the heat resistance necessary in a practical use indicates a rigidity (elastic modulus) of approximately 100 MPa at approximately 80° C.
In consideration of the problem described above, in order to maintain superior properties as a practical material, in recent years, it has been believed that improvement in heat resistance of a resin capable of forming a crystal structure is important.
In order to improve the heat resistance of a biodegradable polyester, for example, a method for adding a heat-resistant inorganic filler, such as talc or mica, has been commonly performed. By this method, the mechanical properties can be improved as well as improvement in heat resistance, and the hardness of the material can also be improved.
However, only by addition of an inorganic filler to a resin, it has been difficult to ensure practically sufficient heat resistance.
Accordingly, heretofore, the crystallization of a poly(lactic acid) was further promoted by performing a heat treatment during or after molding, so that the heat resistance was improved.
Although being a biodegradable polyester capable of forming a crystal structure, a poly(lactic acid) is a polymer which is difficult to be crystallized; hence, when the poly(lactic acid) is molded by a method similar to that for a general-purpose resin, a molded product becomes amorphous or tends to have a high amorphous ratio, so that the mechanical strength is degraded, and the heat distortion is liable to occur.
On the other hand, the crystallization of a material can be promoted by performing a heat treatment during or after molding, and as a result, the heat resistance of a molded product can be improved.
However, since a method for promoting the crystallization by a heat treatment takes a long period of time, the productivity is not superior, thereby causing a practical problem.
When a related general-purpose resin is used, an injection molding step is performed generally in a molding cycle of approximately 1 minute. However, in the case in which a poly(lactic acid) is used, in order to advance the crystallization thereof by performing a heat treatment on a molded product in a mold so as to obtain a practically sufficient mechanical strength, a considerably long time may be necessary as compared to the case in which a general-purpose resin is used.
In addition, in a step of crystallizing a biodegradable polyester, since the spontaneous generation frequency of crystal nuclei is very low, the size of crystals is only on the order of several microns, and white turbidity occurs in a finally obtained resin composition, so that the transparency thereof is degraded. Accordingly, the practical use range is disadvantageously limited.
In order to solve the various problems described above, there has been a method for promoting the crystallization of a biodegradable polyester by adding a nucleating agent to a biodegradable polyester capable of forming a crystal structure.
The nucleating agent described above is an agent which functions as primary crystal nuclei of a crystal polymer and which promotes crystal growth thereof; however, in a broad sense, a material promoting the crystallization of a crystal polymer, that is, a material increasing a crystallization rate itself of a polymer, may also be called a nucleating agent.
When a nucleating agent is added to a biodegradable polyester capable of forming a crystal structure, since an effect of particularizing crystals is obtained, the rigidity of a finally obtained resin composition is improved, and further, the transparency thereof is also improved. In addition, when a nucleating agent is added to a biodegradable polyester capable of forming a crystal structure, since the crystallization rate thereof during molding is improved, the time necessary for an injection molding step can be shortened.
An effect similar to that obtained when a nucleating agent is added to a biodegradable polyester capable of forming a crystal structure is also confirmed in a crystal resin other than a biodegradable polyester.
For example, in the case of polypropylene (hereinafter referred to as “PP” in some cases), by addition of a nucleating agent, improvement in rigidity and improvement in transparency are confirmed. As the nucleating agent used in this case, for example, a sorbitol-based material may be mentioned, and it is believed that a three-dimensional network thereof effectively works on the crystallization of PP. In addition, besides the sorbitol-based material, as a metal salt type material; for example, hydroxy-di(t-butyl benzoic acid)aluminum, bis(4-t-butylphenyl)sodium phosphate, and methylene-bis(2,4-di-t-butylphenyl)phosphate sodium salt may be mentioned.
However, a polyester such as a poly(lactic acid) is used as the resin, since it is not likely to be crystallized as described above, there is a practical problem of a nucleating agent to be used.
For example, when talc having a small nucleating effect is used as a nucleating agent, since a sufficient effect may only be obtained when the addition amount of talc is increased to approximately several tens of percent, the addition amount becomes excessively large. As a result, an obtained resin composition thereby is inferior in mechanical strength, and there has been a problem in that a practically necessary mechanical strength may not be obtained. In addition, when the content of talc is large in the resin, white turbidity occurs, and the transparency is degraded, so that the practical use range may be disadvantageously limited.
Accordingly, for example, in Japanese Unexamined Patent Application Publication No. 10-158369, a technique of a crystallization promoting method has been disclosed in which a sorbitol-based material is applied as a nucleating agent to an aliphatic polyester. In addition, Japanese Unexamined Patent Application Publication No. 10-158369 also has disclosed that by applying a sorbitol-based material as a nucleating agent to a poly(lactic acid), a crystallization effect is obtained.
As another crystallization promoting method by addition of a nucleating agent, for example, in Japanese Unexamined Patent Application Publication Nos. 9-278991 and 11-5849, a technique has been disclosed in which a transparent nucleating agent is applied to an aliphatic polyester. As this transparent nucleating agent, at least one selected from the group consisting of an aliphatic carboxylic acid amide, an aliphatic carboxylic acid salt, an aliphatic alcohol, and an aliphatic carboxylic acid ester and having a melting point of 40 to 300° C. may be used (see Japanese Unexamined Patent Application Publication No. 9-278991). In addition, as another transparent nucleating agent, at least one selected from the group consisting of organic compounds having a melting point or a softening point of 80 to 300° C. and a melting entropy of 10 to 100 cal/K/mol may be used (see Japanese Unexamined Patent Application Publication No. 11-5849).
In addition, in Japanese Unexamined Patent Application Publication No. 11-116783, a technique has also been disclosed in which a fatty acid ester having a specific structure is added as a transparentizing agent to a poly(lactic acid)-based resin.
Furthermore, in Japanese Unexamined Patent Application Publication No. 2004-352872, a technique has been disclosed in which a specific amide-based compound is blended particularly with a poly(lactic acid) to form a resin composition having superior heat resistance and impact strength. In addition, in Japanese Unexamined Patent Application Publication No. 2004-352873, a technique has been disclosed in which a specific heterocyclic compound is blended particularly with a poly(lactic acid) to form a resin composition having superior heat resistance and impact strength. In Japanese Unexamined Patent Application Publication Nos. 2004-352872 and 2004-352873, as the amide-based compound and the heterocyclic compound, phthalic acid hydrazide is described by way of example, and it has been disclosed that in one example in which phthalic acid hydrazide and talc are used in combination, the crystallinity of the poly(lactic acid) can be improved.
In Japanese Unexamined Patent Application Publication No. 2006-282940, a technique has been disclosed in which an amino acid and a polymer capable of forming a crystal structure, in particular, a poly(lactic acid), are contained to manufacture a transparent resin composition having superior rigidity, moldability, and heat resistance. In addition, in Japanese Unexamined Patent Application Publication No. 2006-299091, a technique has been disclosed in which a polymer capable of forming a crystal structure, in particular, a poly(lactic acid), and a material having a specific five-membered ring or six-membered ring are contained to form a transparent resin composition having superior rigidity, moldability, and heat resistance.
Incidentally, it has been believed important for an industrial product formed from polyesters to ensure practical resistance against hydrolysis.
The degree of hydrolysis changes depending on the type of polyester to be used and/or the use environment, and depending on the service period of a molded product, the problem relating to hydrolysis may not become a practical problem. However, when a biodegradable polyester is used, since the problem relating to hydrolysis may become a serious problem in practice, and hence hereinafter, it becomes important to ensure practical durability against hydrolysis.
That is, when the service period is short (short hours), rapid hydrolysis is preferable, and on the other hand, when the service period is long (long hours), the hydrolysis is preferably suppressed.
For example, when a biodegradable polyester is applied to chassis of electrical products, electronic apparatuses, and the like, it is requested to guarantee a long-term reliability for approximately several to ten years, and hence mechanical properties, such as a tensile strength, a flexural strength, and an impact resistance, have to be maintained at a practically sufficient level during the period described above.
As for a technique to improve the long-term reliability of a biodegradable polyester, heretofore, various proposals have been made; however, every proposed technique has failed to simultaneously satisfy improvement in resin crystallinity described above and sufficient material long-term reliability, and a technique capable of satisfying the above desires has not been proposed yet.